Colour micro-LED display apparatus

ABSTRACT

A colour micro-LED display apparatus comprises an array of reflective optical elements and an array of micro-LED pixels with a uniform emission colour across the array arranged between the army of reflective optical elements and an output substrate. Light from the micro-LEDs is directed into the reflective optical elements and is incident on scattering regions in the apparatus. Colour converted scattered light is transmitted by the output substrate. A thin and efficient display apparatus may be provided with high spatial and angular colour uniformity and long lifetime.

TECHNICAL FIELD

The present disclosure relates to colour micro-LED display apparatuscomprising a plurality of addressable micro-LEDs aligned to an array ofreflective optical elements and an array of wavelength conversionelements. Such an apparatus may be used as a high-resolution colourdisplay apparatus.

BACKGROUND

In this specification, (except when qualified by the term packaged),“LED” or micro-LED refers to an unpackaged LED die chip extracteddirectly from a monolithic wafer, i.e. a semiconductor element.Micro-LEDs may be formed by array extraction methods in which multipleLEDs are removed from a monolithic epiaxial wafer in parallel and may bearranged with positional tolerances that are less than 5 micrometres.

This is different from packaged LEDs. Packaged LEDs have a lead-frameand plastic or ceramic package with solder terminals suitable forstandard surface-mount PCB (printed circuit board) assembly. The size ofthe packaged LEDs and limits of PCB assembly techniques means thatdisplays formed from packaged LEDs are difficult to assemble with pixelpitches below about 1 mm. The accuracy of components placed by suchassembly machines is typically about plus or minus 30 micrometres. Suchsizes and tolerances prevent application to very high-resolutiondisplays.

One type of packaged LED display may provide colour pixels by means ofarrays of packaged LEDs emitting in different wavelength bands. Forexample, arrays of packaged red LEDs, arrays of green emitting packagedLEDs and arrays of blue emitting packaged LEDs are soldered on to a PCB.Such displays do not use wavelength conversion layers or colour filtersto achieve colour. The operating voltage of the different colourpackaged LEDs is different from each other for such displays, addingcost and complexity to drive electronics. Further the emissionefficiency of green direct emission is substantially lower than for redand blue emission, reducing display efficiency and brightness.

Liquid crystal displays (LCDs) typically provide colour images by meansof white light backlights and an array of colour filters arranged ateach pixel of the transmissive LCD. Organic LED (OLED) displays providecolour output either through white light emission from each pixel and analigned colour filter or by direct red, green and blue emission fromdifferent OLED materials at each pixel.

Wavelength conversion materials such as phosphors or quantum dotmaterials can absorb light in one wavelength band for example blue andemit light in a different wavelength band, for example yellow. There issome blue transmitted through the phosphor resulting in yellow plus bluewhich appears as white.

Catadioptric optical elements combine refractive surfaces (dioptrics)and reflective surfaces (catoptrics), which may provide total internalreflection or reflection from metallised surfaces. Backlights employingcatadioptric optical elements with small output luminous intensity solidangles are described in WO2010038025 incorporated by reference herein inits entirety.

BRIEF SUMMARY

It would be desirable to provide thin, flexible and free-form shapecolour displays that have high luminance and high efficiency.

According to a first aspect of the present disclosure there is provideda colour display apparatus comprising: a plurality of LEDs arranged inan LED array, wherein the plurality of LEDs are unpackaged micro-LEDs; aplurality of reflective optical elements arranged in a reflectiveoptical element array; and a plurality of wavelength conversion elementsarranged in a wavelength conversion array, wherein each of the pluralityof wavelength conversion elements is arranged to receive light emittedby one or more LEDs of the plurality of LEDs, convert the received lightinto light of a different colour wavelength band, and output the lightof a different colour wavelength band for display, wherein each of theplurality of reflective optical elements is arranged to re-direct atleast part of the light emitted by one or more LEDs of the plurality ofLEDs towards one or more of the plurality of wavelength conversionelements. Advantageously an emissive display may be provided with highresolution, high contrast, high efficiency and low power consumption.The display may be thin, flexible, curved, foldable and have low bezelwidth. Temperature of operation of wavelength conversion materials maybe reduced, increasing efficiency and lifetime. Colour pixels may beprovided with high colour gamut. Low cross talk between pixels may beachieved, to provide high image contrast. The colour output of thedisplay may be substantially independent of viewing angle. Wavelengthconversion materials with particle size similar to LED size may be usedwith high output efficiency, such that for example ground phosphormaterials may be used with micro-LEDs while achieving high imageresolution.

Each of the plurality of wavelength conversion elements may be spacedapart from the one or more LEDs that the wavelength conversion elementis arranged to receive light from.

The LEDs may be arranged to emit light in an opposite direction to adirection in which the wavelength conversion elements output light fordisplay.

The LEDs may be arranged to emit light in the same direction as adirection in which the wavelength conversion elements output light fordisplay.

Each reflective optical element may comprise a reflective rear surfaceand reflective walls extending from the reflective rear surface, thereflective rear surface and reflective walls defining a spacetherebetween.

A transmissive material may be arranged in the space defined by thereflective rear surface and the reflective walls of each reflectiveoptical element. Advantageously dimensional stability may be increasedand susceptibility to pressure and moisture variations reduced.Advantageously the reliability and lifetime of the system is improved.

The reflective rear surface of each reflective optical element maycomprise a reflective light input structure. Each of the plurality ofLEDs may be aligned with a respective reflective light input structure.Advantageously light may be efficiently directed from the micro-LEDincreasing efficiency.

The light emitted by each of the plurality of LEDs may be of the samecolour wavelength band. Advantageously the complexity of the micro-LEDarray fabrication may be reduced, reducing cost. Further the same drivevoltage may be used for the array of micro-LEDs, reducing complexity ofthe control system.

The colour wavelength band of the light emitted by each of the pluralityof LEDs may be blue light.

The colour wavelength band of the light emitted by each of the pluralityof LEDs may be ultraviolet light.

The colour wavelength band of the light emitted by at least one of theLEDs of the plurality of LEDs may be red light and the colour wavelengthband of the light emitted by at least one of the LEDs of the pluralityof LEDs may be blue light or ultraviolet light. Advantageously theefficiency of red light output may be increased and fewer wavelengthconversion elements may be provided, reducing complexity of thewavelength conversion array and reducing cost.

The wavelength conversion elements may comprise a phosphor or a quantumdot material. Advantageously known LED material systems such as galliumnitride may be used to provide high efficiency optical output. Furtherknown wavelength conversion materials may be provided, reducing cost andcomplexity.

The wavelength conversion elements may be formed on the reflective rearsurface of at least some of the reflective optical elements.Advantageously the wavelength conversion elements are remote from themicro-LEDs, reducing operating temperature and increasing efficiency.The area of the wavelength conversion elements may be larger than themicro-LED such that the positional tolerance for the regions may berelaxed, reducing cost.

Some of the reflective optical elements may be not aligned withwavelength conversion elements. Advantageously efficiency of output isincreased. Further the complexity of the array of wavelength conversionelements is reduced, reducing cost.

Some of the reflective optical elements may be aligned with diffuserregions. Advantageously the colour output may be uniform with viewingangle.

The diffuser regions and/or wavelength conversion elements may be formedon a surface of the transmissive material.

Each micro-LED may be an LED that has a width that is at most 150micrometres, preferably at most 100 micrometres and more preferably atmost 50 micrometres. Advantageously a high-resolution display may beprovided with high output efficiency.

The plurality of LEDs may be formed on a surface of the transmissivematerial. Advantageously wavelength conversion materials may be appliedto a surface that may flat by printing or other known depositionmethods.

The colour display apparatus may further comprise an output substrate.Advantageously output light from the reflective optical elements may befurther controlled.

The wavelength conversion elements may be formed on a first side of theoutput substrate and the first side of the output substrate may face thereflective optical element. Advantageously the wavelength conversionelements may be conveniently aligned to the micro-LED array andreflective optical element array, reducing cost. Further the wavelengthconversion elements may be provided at reduced cost.

The colour display apparatus may further comprise a colour filter arraycomprising a plurality of absorptive colour filter regions. Theplurality of colour filter regions may be arranged in an array. Each ofthe colour filter regions may be aligned with only one respectivereflective optical element of the reflective optical element array. Thecolour filter array may be formed on the output substrate.Advantageously, colour gamut may be increased.

Each of the plurality of LEDs may be formed between a respective opticalreflective element and the output substrate.

The plurality of LEDs may be formed on the output substrate.Advantageously, the output substrate material may be provided that isresistant to processing conditions for micro-LED array assembly. Highertemperatures may be provided for micro-LED assembly, advantageouslyincreasing reliability and efficiency. The entire array of micro-LEDsmay be aligned to the entire array of reflective optical elements in asingle alignment step, advantageously reducing cost.

The colour display apparatus may further comprise a control systemarranged to address the plurality of LEDs with display pixel data tocontrol the plurality of LEDs to emit light according to the displaypixel data.

The control system may comprise a plurality of addressing electrodesarranged to provide colour pixel data to each LED of the plurality ofLEDs. Advantageously a colour image may be provided.

The colour display apparatus may further comprise a plurality of lightblocking elements arranged in an array. Each light blocking element maybe aligned with a respective LED of the plurality of LEDs. Therespective aligned LED may be arranged between the light blockingelement and the reflective optical element. Advantageously colour gamutmay be increased by preventing unwanted light propagating directly fromthe aligned micro-LED.

Each light blocking element may be reflective. Advantageously light fromthe micro-LED may be efficiently directed into the reflective opticalelement. Each light blocking element may be an addressing electrode.Advantageously cost and complexity may be reduced.

The output substrate may comprise an optical isolator comprising alinear polariser and at least one retarder. The at least one retardermay be a quarter waveplate. Advantageously unwanted reflections ofambient light from reflective internal surfaces may be reduced,increasing display contrast.

Each LED may be aligned with a respective reflective optical element.

At least one of the LEDs of the plurality of LEDs may be not alignedwith a reflective optical element. Advantageously the complexity of thearray of reflective optical elements may be reduced.

The reflective rear surface of each reflective optical element maycomprise a white reflector. Advantageously a wide-angle output may beprovided. The number fabrication steps may be reduced advantageouslyreducing cost and complexity.

The reflective rear surface of each reflective optical element maycomprise a metal layer. Advantageously the efficiency of output forincident light may be increased.

The reflective rear surface of each reflective optical element maycomprise a planar region. Advantageously light may be redirected withinthe reflective optical element, increasing uniformity of illuminationacross the reflective optical element.

The reflective rear surface of each reflective optical element maycomprise a microstructure. Advantageously the output light may bescattered to provide wide angle luminance profile.

The reflective rear surface of each reflective optical element maydefine a well. A wavelength conversion element may be located within thewell defined by the reflective rear surface of each reflective opticalelement. Advantageously wavelength conversion materials may beconveniently formed on the rear reflective surface. By contrast whenphosphor is applied directly to the LED, the particles of phosphorshould be smaller than the LED. Wavelength conversion particles that arelarge in comparison to micro-LEDs may be provided in the well with highoutput colour uniformity. Advantageously larger wavelength conversionparticles may be cheaper than smaller ones.

A surface of the transmissive material may comprise a refractive inputmicrostructure. The refractive input microstructure may be aligned withthe reflective light input structure. Advantageously light may beefficiently directed from the micro-LED to the reflective rear surface.

In at least one cross-sectional plane the reflective light inputstructure may comprise at least one light deflecting surface. For eachreflective optical element of the reflective optical element array, theat least one light deflecting surface may be arranged to direct at leastsome light from at least one LED towards a wavelength conversionelement. The reflective light input structure may comprise at least onecurved surface. The reflective light input structure may comprise atleast one concave surface. Advantageously light may be efficientlycoupled from the micro-LED to the output of the reflective opticalelement, increasing efficiency and reducing power consumption.

For each reflective optical element of the reflective optical elementarray, the reflective light input structure may be arranged to direct atleast some light from at least one LED to be guided between a surface ofthe transmissive material and the reflective rear surface.Advantageously light may be redistributed within the reflective opticalelement, increasing uniformity of output.

A catadioptric optical element may be aligned with each reflectiveoptical element. The catadioptric optical element may comprise in atleast one catadioptric cross-sectional plane through its optical axis: afirst outer surface and a second outer surface facing the first outersurface. The first and second outer surfaces may comprise curvedsurfaces. The first and second outer surfaces may extend from a firstend of the catadioptric optical element to a second end of thecatadioptric optical element, the second end of the catadioptric opticalelement facing the first end of the catadioptric optical element. Thedistance between the first and second outer surfaces at the first end ofthe catadioptric optical element may be less than the distance betweenthe first and second outer surfaces at the second end of thecatadioptric optical element. At least one transparent inner surface maybe arranged between the first and second ends and between the first andsecond outer surfaces. Advantageously a directional display may beprovided. Privacy display operation may be achieved, that may beswitched between a narrow angle and wide-angle mode of operation.Further power consumption may be reduced for an observer in a head-ondirection. Further stray light may be reduced for off-axis viewingpositions providing low display leakage for night-time operation.

According to a second aspect of the present disclosure there is provideda colour display apparatus comprising: a plurality of LEDs, theplurality of LEDs being arranged in an LED array, wherein the LEDs ofthe plurality of LEDs are micro-LEDs; a control system arranged toaddress the plurality of LEDs with display pixel data to provide inputlight; a reflective optical element array comprising a plurality ofreflective optical elements, the plurality of reflective opticalelements being arranged in an array; wherein each reflective opticalelement of the reflective optical element army comprises: (i) areflective rear surface; (ii) reflective walls extending away from thereflective rear surface (iii) a light transmission opening arrangedbetween the reflective walls and facing the reflective rear surfacewherein each reflective optical element is aligned in correspondencewith a respective one or more LEDs of the plurality of LEDs, the LEDs ofthe plurality of LEDs that are aligned with reflective optical elementsbeing aligned with only a respective one of the reflective opticalelements; a wavelength conversion array comprising a plurality ofwavelength conversion elements, the plurality of wavelength conversionelements being arranged in an array, wherein each of the wavelengthconversion elements is aligned with only a respective one of thereflective optical elements; wherein input light from the respectivealigned at least one LED is input through at least one light inputregion of the light transmission opening and is output through at leastone light output region different from the light input region of thelight transmission opening after incidence on at least one of thereflective rear surface and the respective aligned wavelength conversionelement. Advantageously an emissive display may be provided with highresolution, high contrast, high efficiency and low power consumption.The display may be thin, flexible, curved, foldable and have low bezelwidth. Temperature of operation of wavelength conversion materials maybe reduced, increasing efficiency and lifetime. Colour pixels may beprovided with high colour gamut. Low cross talk between pixels may beachieved, to provide high image contrast. The colour output of thedisplay may be substantially independent of viewing angle. Wavelengthconversion materials with particle size similar to LED size may be usedwith high output efficiency, such that for example ground phosphormaterials may be used with micro-LEDs while achieving high imageresolution.

A transmissive material may be arranged between the reflective rearsurface, the reflective walls and the light transmission opening, andthe light transmission opening comprises a transmissive front surface.Advantageously dimensional stability may be increased and susceptibilityto pressure and moisture variations reduced. Advantageously thereliability and lifetime of the system is improved.

The reflective rear surface may comprise a reflective light inputstructure; wherein the LEDs of the plurality of LEDs that are alignedwith reflective optical elements may be aligned with the reflectivelight input structure. Advantageously light may be efficiently directedfrom the micro-LED increasing efficiency.

The input light from each of the LEDs of the plurality of LEDs may be ofthe same colour wavelength band. Advantageously the complexity of themicro-LED array fabrication may be reduced, reducing cost. Further thesame drive voltage may be used for the array of micro-LEDs, reducingcomplexity of the control system.

The input light colour wavelength band may be blue light or may beultraviolet light. Each of the wavelength conversion elements may bearranged to convert the input light colour wavelength band to visiblelight of a different colour wavelength band. The wavelength conversionelements may comprise a phosphor or a quantum dot material.Advantageously known LED material systems such as gallium nitride may beused to provide high efficiency optical output. Further known wavelengthconversion materials may be provided, reducing cost and complexity.

The input light colour wavelength band from some of the LEDs of theplurality of LEDs may be red light and the input light colour wavelengthband from some of the LEDs of the plurality of LEDs may be blue light orultraviolet light. Advantageously the efficiency of red light output maybe increased and fewer wavelength conversion elements may be provided,reducing complexity of the wavelength conversion array and reducingcost.

The wavelength conversion elements may be formed on the reflective rearsurface of at least some of the reflective optical elements.Advantageously the wavelength conversion elements are remote from themicro-LEDs, reducing operating temperature and increasing efficiency.The area of the wavelength conversion elements may be larger than themicro-LED such that the positional tolerance for the regions may berelaxed, reducing cost.

Some of the reflective optical elements may not be aligned withwavelength conversion elements. Advantageously efficiency of output isincreased. Further the complexity of the array of wavelength conversionelements is reduced, reducing cost.

Some of the reflective optical elements may be aligned with diffuserregions. Advantageously the colour output may be uniform with viewingangle.

The diffuser regions and/or wavelength conversion elements may be formedon the output regions of the transmissive front surface. Advantageouslywavelength conversion materials may be applied to a surface that mayflat by printing or other known deposition methods.

Each micro-LED may be an LED that has a width that is at most 150micrometres, preferably at most 100 micrometres and more preferably atmost 50 micrometres. Advantageously a high-resolution display may beprovided with high output efficiency.

The colour display apparatus may further comprise an output substrate.Advantageously output light from the reflective optical elements may befurther controlled.

The wavelength conversion elements may be formed on a first side of theoutput substrate and the first side of the output substrate may face thelight transmission opening of the reflective optical element.Advantageously the wavelength conversion elements may be convenientlyaligned to the micro-LED array and reflective optical element array,reducing cost. Further the wavelength conversion elements may beprovided at reduced cost.

The colour display apparatus may further comprise a colour filter arraycomprising a plurality of absorptive colour filter regions, theplurality of colour filter regions being arranged in an array. Each ofthe colour filter regions may be aligned in correspondence with a lightoutput region of the light transmission opening of only one of therespective reflective optical elements of the reflective optical elementarray. The colour filter array may be formed on the output substrate.Advantageously, colour gamut may be increased.

The plurality of LEDs may be formed between the transmissive frontsurface and the output substrate. The plurality of LEDs is formed on thetransmissive front surface. Advantageously the complexity of the outputsubstrate may be reduced, reducing cost. The plurality of LEDs may beformed on the output substrate. The output substrate material may beprovided that is resistant to processing conditions for micro-LED arrayassembly. Higher temperatures may be provided for micro-LED assembly,advantageously increasing reliability and efficiency. The entire arrayof micro-LEDs may be aligned to the entire array of reflective opticalelements in a single alignment step, advantageously reducing cost.

The control system may comprise a plurality of addressing electrodesarranged to provide colour pixel data to each LED of the plurality ofLEDs. Advantageously a colour image may be provided.

The colour display apparatus may further comprise a plurality of lightblocking elements, the plurality of light blocking elements beingarranged in an array; wherein each of the light blocking element isaligned in correspondence with a LED of the plurality of LEDs and therespective aligned LED is arranged between the light blocking elementand the reflective optical element for each light blocking element.Advantageously colour gamut may be increased by preventing unwantedlight propagating directly from the aligned micro-LED.

Each light blocking element may be reflective. Advantageously light fromthe micro-LED may be efficiently directed into the reflective opticalelement. Each light blocking element may comprise an addressingelectrode. Advantageously cost and complexity may be reduced.

The output substrate may comprise an optical isolator comprising alinear polariser and at least one retarder. The at least one retardermay be a quarter waveplate. Advantageously unwanted reflections ofambient light from reflective internal surfaces may be reduced,increasing display contrast.

Some of the LEDs of the plurality of LEDs may not be aligned to areflective optical element. Advantageously the complexity of the arrayof reflective optical elements may be reduced.

The reflective rear surface may comprise a white reflector.Advantageously a wide-angle output may be provided. The numberfabrication steps may be reduced advantageously reducing cost andcomplexity.

The reflective rear surface may comprise a metal layer. Advantageouslythe efficiency of output for incident light may be increased.

The reflective rear surface may comprise a planar region. Advantageouslylight may be redirected within the reflective optical element,increasing uniformity of illumination across the reflective opticalelement. The reflective rear surface may comprise a microstructure.Advantageously the output light may be scattered to provide wide angleluminance profile.

The reflective rear surface may comprise a recess or well. The well maycomprise a wavelength conversion element. Advantageously wavelengthconversion materials may be conveniently formed on the rear reflectivesurface. By contrast when phosphor is applied directly to the LED, theparticles of phosphor should be smaller than the LED. Wavelengthconversion particles that are large in comparison to micro-LEDs may beprovided in the well with high output colour uniformity. Advantageouslylarger wavelength conversion particles may be cheaper than smaller ones.

The light input region of the transmissive front surface may comprise arefractive input microstructure. The refractive input microstructure maybe aligned to the reflective light input structure. Advantageously lightmay be efficiently directed from the micro-LED to the reflective rearsurface.

In at least one cross-sectional plane through its optical axis thereflective light input structure may comprise at least one lightdeflecting surface; wherein for each reflective optical element of thereflective optical element array, the at least one light deflectingsurface may be arranged to direct at least some light from therespective aligned at least one LED towards the at least one lighttransmission opening output region. The reflective light input structuremay comprise at least one curved surface. The reflective light inputstructure may comprise at least one concave surface. Advantageouslylight may be efficiently coupled from the micro-LED to the output of thereflective optical element, increasing efficiency and reducing powerconsumption.

For each reflective optical element of the reflective optical elementarray, the reflective light input structure may be arranged to direct atleast some light from the respective aligned at least one LED to guidebetween the transmissive front surface and the reflective rear surface.Advantageously light may be redistributed within the reflective opticalelement, increasing uniformity of output.

A catadioptric optical element may be aligned with the output region ofeach reflective optical element. The catadioptric optical element maycomprise in at least one catadioptric cross-sectional plane through itsoptical axis: a first outer surface and a second outer surface facingthe first outer surface; wherein the first and second outer surfacescomprise curved surfaces; wherein the first and second outer surfacesextend from a first end of the catadioptric optical element to a secondend of the catadioptric optical element, the second end of thecatadioptric optical element facing the first end of the catadioptricoptical element; wherein the distance between the first and second outersurfaces at the first end of the catadioptric optical element is lessthan the distance between the first and second outer surfaces at thesecond end of the catadioptric optical element; and at least onetransparent inner surface is arranged between the first and second endsand between the first and second outer surfaces. Advantageously adirectional display may be provided. Privacy display operation may beachieved, that may be switched between a narrow angle and wide-anglemode of operation. Further power consumption may be reduced for anobserver in a head-on direction. Further stray light may be reduced foroff-axis viewing positions providing low display leakage for night-timeoperation.

Such an apparatus may be used for colour display and for directionaldisplay.

These and other features and advantages of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingfigures, wherein like reference numbers indicate similar parts.

FIG. 1 is a schematic diagram illustrating in side perspective view acolour display apparatus comprising an array of micro-LEDs, an array ofwavelength conversion elements and an array of reflective opticalelements wherein the micro-LEDs are arranged on the reflective opticalelements;

FIG. 2A is a schematic diagram illustrating in side perspective view areflective optical element and an aligned micro-LED arranged to providepixel illumination wherein the reflective optical element comprises adiffusing reflective surface;

FIG. 2B is a schematic diagram illustrating in side view a reflectiveoptical element, a micro-LED and light blocking element;

FIG. 2C is a schematic diagram illustrating in front view a reflectiveoptical element, a micro-LED and light blocking element;

FIG. 3A is a schematic diagram illustrating in side view operation of areflective optical element, a micro-LED and aligned wavelengthconversion element wherein the output substrate is separated from thereflective optical element;

FIG. 3B is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and output wavelength conversionelements wherein a reflective optical element is aligned to each of themicro-LEDs in the array of micro-LEDs;

FIGS. 3C-D are schematic diagrams illustrating in front viewarrangements of wavelength conversion elements and diffusing regions;

FIG. 4 is a schematic diagram illustrating in side view operation of areflective optical element, a micro-LED and aligned wavelengthconversion element wherein a transmissive material is provided betweenthe output substrate and the reflective optical element;

FIG. 5A is a schematic diagram illustrating in side perspective view acolour display apparatus comprising an array of micro-LEDs, an array ofwavelength conversion elements and an array of reflective opticalelements wherein the micro-LEDs are arranged on the output substrate;

FIG. 5B is a schematic diagram illustrating in side perspective view areflective optical element and an aligned micro-LED arranged to providepixel illumination wherein the reflective optical element comprises awavelength conversion element arranged on the reflective surface;

FIG. 6A is a schematic diagram illustrating in side view operation of areflective optical element, a micro-LED and aligned wavelengthconversion element wherein the reflective optical element comprises awavelength conversion element arranged on the reflective surface;

FIG. 6B is a schematic diagram illustrating in side view operation of areflective optical element, a micro-LED and aligned wavelengthconversion element wherein the micro-LED is provided on the transmissivematerial provided between the reflective rear surface and transmissivefront surface.

FIGS. 6C-D are schematic diagrams illustrating in front viewarrangements of colour filters for a colour display apparatus;

FIG. 6E is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged on the reflective surface of some of the reflectiveoptical elements wherein a reflective optical element is aligned to eachof the micro-LEDs in the array of micro-LEDs and the micro-LEDs arearranged to output light of the same colour across the array ofmicro-LEDs;

FIG. 7A is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged on the reflective surface of some of the reflectiveoptical elements wherein a reflective optical element is aligned to eachof the micro-LEDs in the array of micro-LEDs and the micro-LEDs arearranged to output light of the same colour across the array ofmicro-LEDs wherein the area of the micro-LED varies according to theoutput wavelength of the light emitting element and aligned wavelengthconversion element;

FIG. 7B is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged on the reflective surface of the reflective opticalelements wherein a reflective optical element is aligned to some of themicro-LEDs in the array of micro-LEDs and the micro-LEDs are arranged tooutput light of the same colour across the array of micro-LEDs;

FIG. 7C is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged on the reflective surface of some of the reflectiveoptical elements wherein a reflective optical element is aligned to eachof the micro-LEDs in the array of micro-LEDs and the micro-LEDs arearranged to output light of different colours across the array ofmicro-LEDs;

FIG. 7D is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged on the reflective surface of the reflective opticalelements wherein a reflective optical element is aligned to some of themicro-LEDs in the army of micro-LEDs and the micro-LEDs are arranged tooutput light of different colours across the array of micro-LEDs;

FIG. 7E is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged on the reflective surface of some of the reflectiveoptical elements wherein each of a first plurality of reflective opticalelements is aligned to a red emitting micro-LED and a blue emittingmicro-LED and further comprises a light diffusing region; and each of asecond plurality of reflective optical elements is aligned to a blueemitting micro-LED and further comprises a green wavelength conversionelement:

FIG. 7F is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged on the reflective surface of some of the reflectiveoptical elements wherein a reflective optical element is aligned to eachof the micro-LEDs in the array of micro-LEDs and the micro-LEDs arearranged to output ultra-violet light;

FIG. 8A is a schematic diagram illustrating in side view a furtherstructure of reflective optical element, aligned micro-LED and alignedwavelength conversion element wherein the aligned micro-LED is arrangedon an opaque support substrate;

FIG. 8B is a schematic diagram illustrating in side view a furtherstructure of reflective optical element, aligned micro-LED and alignedwavelength conversion element wherein the aligned micro-LED and part ofthe reflective optical element is arranged on an opaque supportsubstrate:

FIG. 9A is a schematic diagram illustrating in side view a furtherstructure of reflective optical element, aligned micro-LED and alignedwavelength conversion element:

FIG. 9B is a schematic diagram illustrating in front view alignment ofreflective optical elements, micro-LEDs and wavelength conversionelements arranged to provide a delta pixel pattern:

FIG. 10A is a schematic diagram illustrating in side view operation of areflective optical element, a micro-LED and aligned wavelengthconversion element wherein the wavelength conversion material comprisesa large particle size material:

FIG. 10B is a schematic diagram illustrating in top view operation of areflective optical element, a micro-LED and aligned wavelengthconversion element wherein the wavelength conversion material comprisesa large particle size material;

FIG. 10C is a schematic diagram illustrating in side view operation of areflective optical element, a micro-LED and aligned wavelengthconversion element wherein the wavelength conversion material comprisesa large particle size material and wherein the micro-LED is provided ona transmissive material provided between the reflective rear surface andtransmissive front surface:

FIG. 11A is a schematic diagram illustrating in front view an array ofreflective optical elements and aligned micro-LED array wherein thewidth of the output of the reflective optical elements transparentoutput region is reduced;

FIG. 11B is a schematic diagram illustrating in top view an army ofreflective optical elements and aligned micro-LED army wherein the widthof the output of the reflective optical elements transparent outputregion is reduced and arranged in alignment to an array of collimatingcatadioptric optical elements;

FIG. 11C is a schematic diagram illustrating in top view viewing of adirectional display by a primary user and an off-axis snooper,

FIG. 11D is a schematic diagram illustrating in top view an array ofreflective optical elements and aligned micro-LED array aligned to anarray of collimating catadioptric optical elements and an array oflinear waveguides to provide a switchable privacy display:

FIG. 11E is a schematic diagram illustrating in top view the structureand operation of a catadioptric optical element and aligned reflectiveoptical element:

FIGS. 12A-C are schematic diagrams illustrating addressing systems forthe plurality of LEDs;

FIG. 13A is a schematic diagram illustrating in side view luminousintensity emission profile from a macroscopic LED:

FIG. 13B is a schematic diagram illustrating in side view luminousintensity emission profile from a micro-LED;

FIG. 14 is a schematic diagram illustrating in side view variation inoptical path length through a wavelength conversion coating for lightfrom a micro-LED;

FIG. 15A is a schematic diagram illustrating in side view output from amicro-LED comprising wavelength conversion coating with large particlesizes:

FIG. 15B is a schematic diagram illustrating in front view output from amicro-LED comprising wavelength conversion coating with large particlesizes:

FIGS. 16A-G are schematic diagrams illustrating in side view a method toform an array of reflective optical elements and aligned micro-LEDarray; and

FIGS. 17A-17F are schematic diagrams illustrating inside view a methodto form an array of reflective optical elements and aligned micro-LEDarray, in accordance with the present disclosure.

DETAILED DESCRIPTION

It would be desirable to provide a micro-LED colour display apparatusthat is a spatial light modulator that is addressed with colour pixelinformation and achieves high luminous efficiency, colour fidelity andcolour uniformity over a wide viewing angle. Further it would bedesirable to provide illumination of wavelength conversion materials ina manner that minimises degradation of performance with time andprotects such materials from environmental degradations.

The structure and operation of various switchable display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

A colour display apparatus comprising an array of micro-LEDs 3 and anarray 120 of wavelength conversion elements will now be described.

FIG. 1 is a schematic diagram illustrating in side perspective view acolour display apparatus 100 comprising an army of micro-LEDs 3, anarray 120 of wavelength conversion elements 20R. 20G and an array 103 ofreflective optical elements 102 wherein the micro-LEDs 3 are arranged onthe reflective optical elements 102; FIG. 2A is a schematic diagramillustrating in side perspective view a reflective optical element 102and an aligned micro-LED 3 of FIG. 1 arranged to provide pixelillumination wherein the reflective optical element 102 comprises adiffusing reflective surface 23 a. 23 b; FIG. 2B is a schematic diagramillustrating in side view a reflective optical element, a micro-LED 3and light blocking element 5; and FIG. 2C is a schematic diagramillustrating in front view a reflective optical element 102, a micro-LED3 and light blocking element 5.

FIG. 1 illustrates a colour display apparatus 100 that comprises: aplurality of LEDs, the plurality of LEDs being arranged in an LED array,wherein the LEDs of the plurality of LEDs are micro-LEDs 3.

The micro-LEDs 3 are unpackaged micro-LEDs, that is they aresemiconductor dies that are extracted directly from a monolithic wafer,i.e. a semiconductor element. Unpackaged micro-LEDs may be formed byarmy extraction methods in which multiple LEDs are removed from amonolithic epitaxial wafer in parallel and may be arranged withpositional tolerances that are less than 5 micrometres. This isdifferent from packaged LEDs. Packaged LEDs have a lead-frame andplastic or ceramic package with solder terminals suitable for standardsurface-mount PCB (printed circuit board) assembly. The size of thepackaged LEDs and limits of PCB assembly techniques means that displaysformed from packaged LEDs are difficult to assemble with pixel pitchesbelow about 1 mm. The accuracy of components placed by such assemblymachines is typically about plus or minus 30 micrometres. Such sizes andtolerances prevent application to very high-resolution displays.

The reflective optical element array 103 may be formed as an integratedbody, such that the army 103 of reflective optical elements 102 providea substrate on which the micro-LEDs 3 are formed. In the embodiment ofFIGS. 1-2C the micro-LEDs 3 are in a micro-LED array formed on theintegrated body with the reflector array. In other embodiments describedbelow the micro-LEDs may be formed on a separate substrate which isaligned with the array 103 of reflective optical elements 102. Themicro-LEDs 3 may be transferred by parallel transfer methods asdescribed below to the reflective optical element array 103. Thus thealignment between the micro-LEDs 3 and reflective optical element array103 may be achieved in a single step or a small number of steps asopposed to aligning each individual reflector with each individualmicro-LED. The position of the micro-LEDs 3 and reflective opticalelements 102 may be defined or formed with lithographic precision sothat the array alignment may be achieved with high precision.Advantageously uniformity is increased and alignment time and costs arereduced. The wavelength conversion materials may be provided with lowerprecision compared to that provided for the micro-LEDs and reflectiveoptical elements. Advantageously lower cost methods may be provided forforming the array 120 of wavelength conversion elements 20.

The colour display apparatus thus comprises: a plurality of LEDs thatare unpackaged LEDs arranged in an LED array, wherein the plurality ofLEDs are micro-LEDs 3; a plurality of reflective optical elements 102arranged in a reflective optical element array 103; and a plurality ofwavelength conversion elements 20 arranged in a wavelength conversionarray 120, wherein each of the plurality of wavelength conversionelements 20 is arranged to receive light emitted by one or more LEDs 3of the plurality of LEDs, convert the received light into light of adifferent colour wavelength band, and output the light of a differentcolour wavelength band for display, wherein each of the plurality ofreflective optical elements 102 is arranged to re-direct at least partof the light emitted by one or more LEDs 3 of the plurality of LEDstowards one or more of the plurality of wavelength conversion elements20. Each of the plurality of wavelength conversion elements 20 is spacedapart from the one or more LEDs 3 that the wavelength conversion element20 is arranged to receive light from.

In a large size TV application colour sub-pixel pitch may be typicallyof order 200×600 micrometres, with a full colour pixel pitch of 600×600micrometres for an RGB stripe pixel arrangement. Micro-LED 3 sizes ofapproximately 10) micrometres or less may be typically provided for eachcolour sub-pixel. In mobile displays such as for a cell phoneapplication, colour sub-pixel pitch may be typically of order 20×60micrometres. For such pixels micro-LED 3 sizes of 10 micrometres or lessmay be provided. In the present disclosure, a micro-LED 3 is an LED thathas a width or diameter that is at most 150 micrometres, preferably atmost 100 micrometres and more preferably at most 50 micrometres. Themicro-LEDs 3 of the array of micro-LEDs may be square, rectangular orother shapes such as circular.

A control system is arranged to address the plurality of micro-LEDs 3with display pixel data to provide input light and comprises a pluralityof addressing electrodes 210, 212 arranged to provide colour pixel datato each micro-LED 3 of the plurality of micro-LEDs 3. The control systemfurther comprises row and column electrode drivers 202, 204 and acontroller 200 arranged to provide image data to the drivers 202, 204.The control system may also comprise additional circuitry including butnot limited to thin film transistors, capacitors, ICs or transistorslocated within the array of micro-LEDs 3 and adjacent to the micro-LEDs3.

In the present embodiments, the area of the micro-LEDs 3 may besubstantially less than the total pixel area defined by pixel pitch.Further electronics, for example for touch sensing may be providedbetween the micro-LEDs 3. Controller 200 may also be arranged to processand sense measurement data from touch sensors (not shown).

A reflective optical element array 103 comprises a plurality ofreflective optical elements 102, the plurality of reflective opticalelements 102 being arranged in an array. The reflective optical elementmay be formed in or on an optical body 47. Transmissive material 40 maybe provided between the reflective rear surface 43 and transmissivefront surface 33.

The colour display apparatus 100 further comprises an output substrate52 that is transmissive and arranged to receive light from thereflective optical element array 103. The plurality of micro-LEDs 3 isformed between the transmissive front surface 33 and the outputsubstrate 52.

As illustrated further in FIGS. 2A-C, each reflective optical element102 of the reflective optical element array 103 comprises: (i) areflective rear surface 43 (ii) reflective walls 49 extending away fromthe reflective rear surface 43 and (iii) a light transmission opening133 arranged between the reflective walls 49 and facing the reflectiverear surface 43.

Transmissive material 40 is arranged between the reflective rear surface43, the reflective walls 49 and the light transmission opening 133, andthe light transmission opening comprises a transmissive front surface33.

The reflective rear surface 43 further comprises a reflective lightinput structure 44; wherein the micro-LEDs 3 of the plurality of LEDsthat are aligned with reflective optical elements 102 are aligned withthe reflective light input structure 44 for example through optical axis111 that is centred on the reflective optical element 102 and micro-LED3.

In other words, each reflective optical element 102 of the reflectiveoptical element array 103 comprises (i) a transmissive front surface 33comprising a light input region 35 and at least one light output region37 a, 37 b; (ii) a reflective rear surface 43 facing the transmissivefront surface 33 comprising a reflective light input structure 44wherein the reflective light input structure 44 is aligned with thelight input region 35; (iii) reflective walls 49 extending between thereflective rear surface 43 and the transmissive front surface 33. Thewalls 49 of neighbouring optical elements 102 are separated by the uppersurface 45 of body 47.

As illustrated in FIG. 2C, adjacent reflective optical elements 102 a,102 b, 102 c may be provided with independently addressed micro-LEDs 3a, b, 3 c. Thus, reflective optical elements 102 a, 102 b, 102 c may beaddressed with red, green and blue pixel data respectively. Furtherreflective optical elements 102 (not shown) that are addressed with datafor further colours such as yellow and white may be further provided.

Each reflective optical element 102 is aligned in correspondence with arespective one or more micro-LEDs 3 of the plurality of micro-LEDs, themicro-LEDs 3 of the plurality of micro-LEDs that are aligned withreflective optical elements 102 being aligned with only a respective oneof the reflective optical elements 102.

The light input region 35 of each reflective optical element 102 isaligned in correspondence with a respective one or more micro-LEDs 3 a,3 b, 3 c of the plurality of micro-LEDs 3, each of the micro-LEDs 3 ofthe plurality of micro-LEDs 3 that are aligned with reflective opticalelements 102 a, 102 b, 102 c being aligned with only a respective one ofthe reflective optical elements 102 a, 102 b, 102 c.

The colour display apparatus 100 further comprises a plurality of lightblocking elements 5, the plurality of light blocking elements 5 beingarranged in an array. Each of the light blocking elements 5 is alignedin correspondence with a micro-LED 3 of the plurality of micro-LEDs 3and the respective aligned micro-LED 3 is arranged between the lightblocking element 5 and the reflective optical element 102 for each lightblocking element 5. The light blocking element may also function as acomponent such as an electrode or capacitor plate in the addressing ofthe micro-LEDs 3.

In operation, micro-LED 3 is arranged to input input light rays 110through the light input region 35 of the transmissive front surface 33.Light blocking element 5 may be reflective and may be arranged to blockor to reflect light rays from the micro-LED 3 that are otherwisedirected away from the light input region 35. In other words, the lightoutput from the micro-LED 3 is directed away from the output directionof the display 100.

Each light blocking element 5 may also be an addressing electrode, forexample connected to electrode 210. The light blocking element 5 mayprovide a large area attachment electrode for the micro-LED,advantageously achieving convenient tolerance for micro-LED 3 mountingand for connection to the addressing electrodes 210 or 212.

The reflective mar surface 43 and reflective walls 49 may comprise areflective coating 41 that may be a metal coating or a white reflectivecoating. Alternatively, the material of the optical body 47 may be awhite material such as CEL-W epoxy material marketed by HitachiChemical, such that reflective coating 41 may be omitted, advantageouslyreducing cost and fabrication complexity.

As illustrated in FIG. 2B, the reflective rear surface 43 may furthercomprise a microstructure in light diffusing regions 23 comprisingsurface relief structures 46 and may further comprise planar regions 48.

In at least one cross-sectional plane the reflective light inputstructure 44 comprises a first light deflecting surface 64 a and asecond light deflecting surface 64 b wherein the first and second lightdeflecting surfaces 64 a, 64 b are arranged to direct to first andsecond regions of the reflective optical element 102.

In operation, some of the input light rays 110 from micro-LED 3 areincident on reflective light input structure 44 and are deflected so asto be incident onto light diffusing regions 23 a. 23 b as will bedescribed further below.

Input light rays 110 that are not incident on input structure 44 areincident directly on the light diffusing regions 23 a, 23 b of thereflective rear surface 43.

Light rays 110 incident on microstructure 46 of the light diffusingregions 23 a. 23 b of the reflective rear surface 43 are diffused asoutput light such that light rays 112 are transmitted through the lightoutput region 37 a, 37 b of the transmissive front surface 33. As willbe described below, some light rays 113 may be guided within thereflective optical element 113. Further some light rays 114 may bereflected by reflective walls 49 that may be diffusive or planar.

The light blocking layer 5 prevents light output from the input region35 of the transmissive front surface and the input structure 44efficiently directs input light onto the light diffusing regions 23 a.23 b. Advantageously the output regions 37 a. 37 b of the transmissivefront surface 33 are efficiently filled with light from the micro-LED.

Light from micro-LED 3 b is constrained within a single reflectiveoptical element 102 b before output at transmissive front surface 33output region 35. The light output from each reflective optical element102 b is optically separated from adjacent elements 102 a, 102 c.Advantageously cross talk between adjacent optical outputs is reducedand high contrast may be achieved.

Light ray propagation in the display apparatus 100 will now be furtherdescribed.

FIG. 3A is a schematic diagram illustrating in side view operation of areflective optical element 102, a micro-LED 3 and aligned wavelengthconversion array 120 wherein the output substrate 52 is separated fromthe reflective optical element 102.

Gap 50 is provided between the reflective optical element 102 and outputsubstrate 52.

Light output from the micro-LED 3 is typically Lambertian, that is thehighest luminous intensity of output is in the normal direction(parallel to the z direction).

The reflective light input structure 44 comprises a curved surface thatis a concave surface. In operation light rays that are emitted in thenormal direction from the micro-LED are directed towards the lightdiffusing regions 23 and not towards the micro-LED 3 and light blockingelement 5. By way of comparison, when the reflective light inputstructure 44 is planar, then light will be reflected back towards themicro-LED 3 and light blocking element 5. Advantageously the reflectivelight input structure 44 increases display light output efficiency.

For each reflective optical element 102 of the reflective opticalelement array 103, the reflective light input structure 44 is arrangedto direct at least some light rays 110 from the respective aligned atleast one micro-LED 3 to guide as light rays 116 between thetransmissive front surface 33 and the reflective rear surface 43 in thetransmissive material 40. Planar regions 48 on the reflective rearsurface 43 between microstructure 48 elements provide guiding of lightfrom the reflective rear surface 43. Light rays 110 are distributed overthe light diffusing regions 23 a, 23 b. Advantageously such distributionof light in the reflective optical element 102 reduces the peak energydensity of light at the wavelength conversion elements and achieveshigher efficiency and increased lifetime as will be further describedbelow.

FIG. 3A further illustrates light ray propagation in the outputsubstrate 52. Wavelength conversion array 120 is arranged on the inputside of a transparent support substrate 53. Light rays 112 from theoutput regions 37 of the transmissive front surface 33 of the reflectiveoptical element 102 incident on wavelength conversion element of array120 an undergo wavelength conversion at wavelength conversion location24. An optical isolator comprising retarder 54 and polariser 56 isarranged on the output side of transparent support substrate 53,operation of which will be described further below.

The output substrate 52 comprises an optical isolator comprising alinear polariser and at least one retarder. The at least one retarder isa quarter waveplate.

The upper surface 45 of body 47 may further be provided with a coating145. The coating may be provided in the same step used to providereflective coating 41, advantageously reducing cost. The opticalisolator removes visibility of reflections from the surface 45.Alternatively, an absorbing coating 145 may be provided on the uppersurface 45 of body 47. The array 120 of wavelength converting elements20 may further be provided with an absorbing mask 25. Advantageouslystray light from the reflective optical elements may be prevented frompropagating between adjacent reflective optical elements, improvingcross talk.

Features of the arrangement of FIG. 3A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIG. 3B is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, micro-LEDs 3 and output wavelengthconversion elements 20R, 20G and diffuser element 21 of FIG. 1 wherein areflective optical element 102 is aligned to each of the micro-LEDs 3 inthe array of micro-LEDs 3.

The wavelength conversion array 120 comprises a plurality of wavelengthconversion elements 20R, 20G, the plurality of wavelength conversionelements 2R, 20G being arranged in an array, wherein each of thewavelength conversion elements 20R, 20G is aligned with only arespective one of the reflective optical elements 102. Each of thewavelength conversion elements 20R, 20G is aligned in correspondencewith a light output region 37 of the transmissive front surface 33 ofthe respective reflective optical element 102.

In the present disclosure a wavelength conversion element 20 is a regioncomprising a wavelength conversion material. As will be describedfurther below, the wavelength conversion elements 20 may be arranged onor in the reflective optical elements 102 or may be arranged on outputsubstrate 52 as illustrated in FIG. 3A.

Electrodes 211, 213 are arranged to connect micro-LED 3 and lightblocking element 5 via addressing circuitry such as TFTs (not shown) ordirectly to row and column addressing electrodes 210, 212 respectively.

In an illustrative embodiment the input light 110 from each of themicro-LEDs 3 of the plurality of micro-LEDs 3 is of the same colourwavelength band that is blue light. Advantageously the micro-LEDs may beprovided from gallium nitride wafers across the array of micro-LEDs. Theforward voltage characteristics of the array may be the same for all themicro-LEDs 3, reducing cost and complexity of the control system.

The wavelength conversion elements 20R, 20G are formed on a first sideof the output substrate 52 and the first side of the output substrate 52faces the transparent front surface of the reflective optical element102.

Each of the wavelength conversion elements 20R, 20G is arranged toconvert input light 110 to visible light of a different wavelength bandto the wavelength band of the input light 110. The wavelength conversionelements 20R, 20G may comprise a phosphor or a quantum dot material.Efficient colour conversion materials that have been tuned for use withgallium nitride emission wavelengths may be used, advantageouslyreducing cost and power consumption.

Input light rays 110 from the respective aligned at least one micro-LED3 is input through the light input region 35 of the transmissive frontsurface 33 and is output through at least one light output region 37 ofthe transmissive front surface 33 after incidence on at least one of thereflective rear surface 43. The output light 112 is incident onwavelength conversion array 120 and the respective wavelength conversionelement 20R, 20G. At wavelength conversion location 24 light rays 112undergo wavelength conversion and output light rays 118 are scattered.Some light rays 119 are back scattered and undergo reflection from thereflective rear surface 43 of the aligned reflective optical element 102and are output as output rays.

Advantageously an addressable colour sub-pixel may be provided by eachreflective optical element 102. Further the energy density at thewavelength conversion material is substantially lower than the energydensity if the wavelength conversion material were to be provideddirectly on the micro-LED 3. Further the temperature of operation of thewavelength conversion material is reduced, advantageously increasingconversion efficiency and material lifetime. Further a uniform angularillumination of the wavelength conversion element 20 is provided suchthat advantageously colour variations with angle are minimised.

In the case that the input light is blue light, it may not be desirableto provide wavelength conversion. Some of the reflective opticalelements 102 are aligned with diffuser regions 21. In combination withthe diffusing regions 23 of the reflective rear surface 43 of thereflective optical elements 102, diffuser regions 21 provide similarscattering properties to the light that is scattered by the wavelengthconversion elements 20R, 20G. Advantageously uniform colour may beprovided from a wide range of viewing angles.

Features of the arrangement of FIG. 3B not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIGS. 3C-D are schematic diagrams illustrating in front viewarrangements of wavelength conversion elements 20R, 20G and diffusingregions 21.

The wavelength conversion elements 20R, 20G and diffusing regions 23 maybe arranged as continuous stripes as illustrated in FIG. 3C, and thewavelength conversion elements for adjacent reflective optical elementsmay be contiguous in one direction. Each stripe of contiguous wavelengthconversion elements 20R, 20G are arranged with a column of reflectiveoptical elements 102.

Advantageously, such an arrangement may be conveniently provided byprinting or other known deposition methods onto support substrate 53.The angular diffusion properties of region 21 may be provided to matchthe diffusion produced by wavelength conversion elements 20A, 20B.

FIG. 3D illustrates that wavelength conversion elements 20R, 20G anddiffusing regions 21 may be provided with light absorbing mask 25 may beprovided between the regions 20A, 20G, 21. Advantageously cross talkbetween adjacent colour sub-pixels may be provided, as stray light maybe absorbed in light absorbing mask 25.

FIG. 4 is a schematic diagram illustrating in side view operation of areflective optical element 102, a micro-LED 3 and aligned wavelengthconversion element wherein a transmissive material 51 is providedbetween the output substrate 52 and the reflective optical element 102.

Further, for each reflective optical element 102 of the reflectiveoptical element array 103, the reflective light input structure 44 isarranged to direct at least some light from the respective aligned atleast one micro-LED 3 towards the at least one transmissive frontsurface 33 output region 37.

Diffuser regions 21 and/or wavelength conversion elements 20R, 20G areformed on the output regions 37 of the transmissive front surface 33.Output substrate 52 may be attached to the array 103 of reflectiveoptical elements 102 by means of material 51 that may be a transparentbonding material.

Further micro-LEDs 3 and addressing electrodes 210, 212 may be arrangedon the front surface 33 of the reflective optical elements 102.Advantageously rugged alignment of micro-LEDs 3 and reflective opticalelements 102 may be achieved.

Advantageously an integrated optical structure may be provided withoutair gaps. Internal reflections and stray light may be reduced. Further,sensitivity to environmental pressure changes may be reduced, andalignment may be maintained between components of the display fortemperature and pressure changes. In comparison to FIG. 3A, parallaxbetween the wavelength conversion array 120 and reflective opticalelements 102 may be reduced, achieving improved cross-talk.

Features of the arrangement of FIG. 4 not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

A further arrangement of the array of micro-LEDs 3 and array 103 ofreflective optical elements 102 will now be described.

FIG. 5A is a schematic diagram illustrating in side perspective view acolour display apparatus 100 comprising an array of micro-LEDs 3arranged on an output substrate 52, an array of wavelength conversionelements 20R, 20G, array of light diffusing regions 21 and an array ofreflective optical elements 102. The plurality of micro-LEDs 3 is formedon the output substrate 52.

Advantageously, the substrate 52 may comprise a material suitable forforming addressing electrodes 210, 212, and for attaching micro-LEDs 3and other addressing electronics. For example, the substrate 52 maycomprise a glass support substrate that can be processed with hightemperature in comparison to a polymeric material that may be used toform the body 47 for the reflective optical elements 102. For example,micro-LED 3 may be attached to substrate 52 by means of high temperaturesolders. Alternatively, output substrate 52 may comprise a supportsubstrate 53 that comprises a flexible material for example a polymersuch as polyimide that can be used with electrode deposition techniquesdeveloped for flexible film OLED displays.

Features of the arrangement of FIG. 5A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

A further arrangement of reflective optical element 102 will now bedescribed.

FIG. 5B is a schematic diagram illustrating in side perspective view areflective optical element 102 and an aligned micro-LED 3 arranged toprovide pixel illumination wherein the reflective optical element 102comprises a wavelength conversion element 20R arranged on the reflectiverear surface 43.

FIG. 5B differs from FIG. 2A in that the light input region 35 of thetransmissive front surface 33 comprises a refractive inputmicrostructure 72. Input microstructure 72 redistributes the outputlight cone from the micro-LED 3 such that input light rays 110 frommicro-LED 3 are directed in directions that are towards the diffusingregion 23 of the reflective rear surface 43. Advantageously efficiencyof output may be increased.

FIG. 5B further illustrates that the reflective rear surface 43comprises a wells 70 a, 70 b. The wavelength conversion elements 20R,20G are formed on the reflective rear surface 43 of at least some of thereflective optical elements 102 and is formed in the wells 70 a, 70 b.The wells 70 a, 70 b comprise a wavelength conversion element 20R. Inother words, the material of the wavelength conversion element 20R isarranged in the wells 70 a, 70 b. Alternatively wells 70 a and 70 b maycontain different wavelength conversion materials for example twodifferent red phosphors in order to produce a wider colour gamut for thedisplay.

Features of the arrangement of FIGURE SA not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIG. 6A is a schematic diagram illustrating in side view operation of areflective optical element 102, a micro-LED 3 and aligned wavelengthconversion element 20 wherein the reflective optical element 102comprises a wavelength conversion element arranged on the reflectiverear surface 43. The wells 70 a, 70 b may be provided in addition to oralternatively to the light diffusing regions 23 a, 23 b of FIG. 2B.Thus, the wells 70 a, 70 b may be provided with wavelength conversionmaterials such as quantum dot or phosphor materials.

Transmissive material 40 that is arranged between the wavelengthconversion materials in region 20R and the transparent front surface 33may additionally provide a protective coating for the material of thewavelength conversion materials. Advantageously material 40, reflectivecoating 41, the material arranged to form the body 47 and outputsubstrate 52 may provide barriers to water and oxygen migration to thewavelength conversion materials. Further barrier layers 75 that maycomprise inorganic coatings such as silica oxides or aluminium oxidesmay be provided on the substrate 52 and body 47. Advantageouslyefficiency of output may be increased and lifetime of materialsextended.

The reflective optical element 102 array 103 may comprise an integratedbody that is aligned to the substrate 52 on which the micro-LEDs 3 areformed. The integrated body of the array 103 and substrate 52 comprisingmicro-LEDs 3 may be aligned in a single step as opposed to multipleindividual alignments. Advantageously alignment time and costs amreduced.

In operation, input light rays 110 from the respective aligned at leastone micro-LED 3 is input through the light input region 35 of thetransmissive front surface 33 and is output as light rays 118 through atleast one light output region 37 of the transmissive font surface 33after incidence on at least one reflective rear surface 43 and therespective wavelength conversion element 20R. 20G.

For blue pixels, diffuser regions 23 may be provided in place ofwavelength conversion elements 20R, 20G. The diffuser regions may beprovided by surface relief structure as illustrated in FIG. 2B forexample or may be provided by a bulk diffuser, such as suspended whitelight scattering particles such as titanium dioxide. The bulk diffusermay be arranged in a curable material and arranged in the wells 70 ofthe reflective optical element 102 for example. Both bulk diffusion andsurface relief diffusion comprising microstructure 46 arranged on thereflective surface 43 may be provided.

For colour converted pixels, some blue light may be scattered from redand green pixels, undesirably reducing colour gamut.

It would be desirable to increase colour gamut.

The colour display apparatus 100 further comprises a colour filter array122 comprising a plurality of absorptive colour filter regions, theplurality of colour filter regions 22 being arranged in an array;wherein each of the colour filter regions is aligned in correspondencewith a light output region 37 of the transmissive front surface 33 ofonly one of the respective reflective optical elements 102 of thereflective optical element array 103.

Colour filter array 122 may be provided on the input surface of theoutput substrate 52 or may be formed on the output regions 37 of thetransmissive front surface 33. Colour filter array 122 may transmitlight rays 112 that are colour converted by interaction with thematerial of the wavelength conversion element 20 in the respective well70.

Features of the arrangement of FIG. 6A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIG. 6B is a schematic diagram illustrating in side view operation of areflective optical element 102, a micro-LED 3 and aligned wavelengthconversion element 20 wherein the micro-LED 3 is provided on thetransmissive material 40 provided between the reflective rear surface 43and transmissive front surface 33. In comparison to the arrangement ofFIG. 6A, the micro-LED may be aligned on the reflective optical element102 during fabrication, and further alignment steps are reduced,advantageously reducing cost and complexity. Features of the arrangementof FIG. 6B not discussed in further detail may be assumed to correspondto the features with equivalent reference numerals as discussed above,including any potential variations in the features.

FIGS. 6C-D are schematic diagrams illustrating in front viewarrangements of colour filters 22A, 22B for the colour display apparatus100 of FIG. 6A.

As illustrated in FIG. 6C colour filter array 122 may comprise clearregions for blue pixels, green transmitting filters 22A for green pixelsand red transmitting filters 22B for red pixels. Colour leakage isreduced and advantageously colour gamut may be increased.

FIG. 6D illustrates that a further blue absorbing colour filter 22B maybe provided. Advantageously red and green light leakage due to straylight in the blue sub-pixels is reduced.

The arrangement of wavelength conversion elements will now be furtherdescribed.

FIG. 6E is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, micro-LEDs 3 and wavelength conversionelements 20R. 20G arranged on the reflective rear surface 43 of some ofthe reflective optical elements 102 wherein a reflective optical element102 is aligned to each of the micro-LEDs 3 in the array of micro-LEDs 3and the micro-LEDs 3 are arranged to output light of the same colouracross the array of micro-LEDs 3.

Some of the reflective optical elements 102 are not aligned withwavelength conversion elements 20R, 20G, and diffusers 23 such as bulkor surface relief diffusers are provided for example as described withrespect to FIG. 6A. The diffuser regions 23 may be replaced by colourfilter 23B so that the blue colour of for example the blue emittingmicro-LEDs 3 may be modified.

Advantageously a high efficiency colour display with long lifetimematerials and high colour gamut may be provided.

Features of the arrangement of FIG. 6E not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Various other arrangements of micro-LED 3 array and reflective opticalelement 102 will now be described.

FIG. 7A is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, micro-LEDs 3 and wavelength conversionelements 20R, 20G arranged on the reflective rear surface 43 of some ofthe reflective optical elements 102 wherein a reflective optical element102 is aligned to each of the micro-LEDs 3 in the array of micro-LEDs 3and the micro-LEDs 3 are arranged to output light of the same colouracross the army of micro-LEDs 3 wherein the area of the micro-LED 3varies according to the output wavelength of the light emitting elementand aligned wavelength conversion element.

In comparison to FIG. 6E, the size of the micro-LEDs for red, green andblue sub-pixels is different. The efficiency of luminous output can varybetween red, green and blue sub-pixels. Different sized micro-LEDs 3 a,3 b, 3 c may provide compensation for the differences in luminousefficiency of the red, green and blue colour sub-pixels. A largermicro-LED 3 may provide higher luminous flux for the same drive voltagethan a smaller micro-LED. Advantageously the control system may besimpler with lower complexity and cost.

FIG. 7B is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, micro-LEDs 3 and wavelength conversionelements 20R, 20G arranged on the reflective rear surface 43 of thereflective optical elements 102 wherein a reflective optical element 102is aligned to some of the micro-LEDs 3 in the army of micro-LEDs 3 andthe micro-LEDs 3 are arranged to output light of the same colour acrossthe array of micro-LEDs 3.

Some of the micro-LEDs 3B of the plurality of micro-LEDs are not alignedto a reflective optical element 102 and further have no light blockingelement 5. Light may be output directly through output substrate 52without being incident onto surface 45 of body 47 by means of areflector arranged between the micro-LED 3B and the body 47.Alternatively, the surface 45 of body 47 may comprise a reflectivecoating 41 such that light from the micro-LED 3 is reflected anddirected through the output substrate 52.

For example, blue sub-pixels may be provided directly by micro-LEDs 3Cand reflective optical elements and wavelength conversion elements 20R,20G may provide red and green light from blue input light frommicro-LEDs 3R,3G,3B. Using the micro-LEDs of the same colour, forexample blue means that each display colour channel for example R, G, Bmay be addressed by the same voltage. Advantageously, the cost andcomplexity of the array 103 of reflective optical elements may bereduced.

FIG. 7C is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, red emitting micro-LEDs 3R, blueemitting micro-LEDs 3G with green wavelength conversion elements 20G andblue emitting micro-LEDs 3B. Green wavelength conversion elements 20Gare arranged on the reflective rear surface 43 of some of the reflectiveoptical elements 102 wherein a reflective optical element 102 is alignedto each of the micro-LEDs 3R, 3G, 3B in the array of micro-LEDs and themicro-LEDs 3R, 3G, 3B are arranged to output light of different coloursacross the array of micro-LEDs.

The input light 110 from some of the micro-LEDs 3R of the plurality ofmicro-LEDs 3 is red light and the input light 110 from some of themicro-LEDs 3G, 3B of the plurality of micro-LEDs is blue light.

Advantageously the optical output distribution may be substantially thesame for each colour pixel, and increased luminous efficiency may beprovided for the red sub-pixels in comparison to wavelength convertinglight from blue to red. The forward voltage difference between drivingthe red micro-LEDs 3R and blue micro-LEDs 3B may be compensatedcontrolling the micro-LED 3 current and or adjusting the drive voltagefor different colour columns of micro-LEDs 3.

FIG. 7D is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, red emitting micro-LEDs 3R, blueemitting micro-LEDs 3G with green wavelength conversion elements 20G andblue emitting micro-LEDs 3B. Green wavelength conversion elements 20Gare arranged on the reflective rear surface 43 of the reflective opticalelements 102 which are aligned to the micro-LEDs 3G in the array ofmicro-LEDs and the micro-LEDs 3R are arranged to output light ofdifferent colours to micro-LEDs 3G, 3B across the array of micro-LEDs.

Thus, colour converted sub-pixels may be illuminated by blue micro-LEDs3B that are converted by wavelength conversion element 20G to greenlight, whereas blue and red pixels are provided by blue and redmicro-LEDs 3B. 3R with no reflective optical element 102. Advantageouslya less complex arrangement is provided, reducing cost.

FIG. 7E is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, micro-LEDs 3R, 3G, 3B and wavelengthconversion elements 20G on the reflective rear surface 43 of some of thereflective optical elements 102 wherein each of a first plurality ofreflective optical elements 102 is aligned to a red emitting micro-LED3R and a blue emitting micro-LED 3B and further comprises a lightdiffusing region 21; and each of a second plurality of reflectiveoptical elements 102 is aligned to a blue emitting micro-LED 3G andfurther comprises a green wavelength conversion element 20G.

In comparison to the arrangement of FIG. 7A, the resolution of thedisplay is advantageously increased, as some of the colour reflectiveoptical elements 102 may provide two different addressable colours.Further for a given pixel pitch, increased feature size may be provided,advantageously reducing cost and complexity of tooling and replication.

FIG. 7F is a schematic diagram illustrating in front view alignment ofreflective optical elements 102 and aligned ultra-violet emittingmicro-LEDs 3R, 3G, 3B that are not red, green and blue light emittingmicro-LEDs but are ultra-violet micro-LEDs that are addressed with red,green and blue pixel data. Wavelength conversion elements 20R, 20G, 20Bare arranged on the reflective rear surface 43 of the respectivereflective optical elements 102 wherein a reflective optical element 102is aligned to each of the micro-LEDs 3R, 3G, 3B in the array ofmicro-LEDs and the micro-LEDs 3R, 3G, 3B are arranged such that theinput light 110 is ultra-violet light.

In comparison to the arrangement of FIG. 7A, a blue wavelengthconversion element 20B is provided to convert ultraviolet light to bluelight. Advantageously, a uniform array of micro-LEDs is providedreducing cost and complexity of assembly and addressing. Further leakagebetween pixels of input light 110 can be minimised by providing anultra-violet filter on or in the output substrate 52. Cross talk maythereby be reduced. There is no visible mixing of the input wavelengthband of light with the wavelength converted band of output light and thecolour gamut of the display is improved.

Features of the arrangements of FIGS. 7A-F not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

FIG. 7F further illustrates an arrangement of electrodes 212, 213, 210and drive circuit 820 for addressing the micro-LEDs 3R, 3G, 3B as willbe described further below.

It may be desirable to provide the micro-LEDs on an opaque supportsubstrate.

FIG. 8A is a schematic diagram illustrating in side view a furtherstructure of reflective optical element 102, aligned micro-LED 3 andaligned wavelength conversion elements 20 wherein the aligned micro-LED3 is arranged on an opaque support substrate 155. Transparent substrate153 is provided with retarder 54, polariser 56 and optionally colourfilters as described elsewhere herein. Features of the arrangement ofFIG. 8A not discussed in further detail may be assumed to correspond tothe features with equivalent reference numerals as discussed above,including any potential variations in the features.

In comparison to arrangements with a transparent support substrate 53,light emission from the micro-LED is provided in the same direction asthe output substrate 52. Reflective light input structure 44 providestotal internal reflection for light rays 110 from the micro-LED 3 suchthat rays are guided on to the wavelength conversion elements 20 andoutput surface 33.

The substrate 155 may be provided with opaque electrode materials andother control electronic components that do not reduce the outputefficiency. Advantageously efficiency may be increased. Further thesubstrate 155 may be provided with thermally conductive regions and/orlayers arranged to achieve reduced semiconductor junction temperatureduring operation of the micro-LED. Advantageously micro-LED efficiencymay be increased.

FIG. 8B is a schematic diagram illustrating in side view a furtherstructure of reflective optical element, aligned micro-LED and alignedwavelength conversion element wherein the aligned micro-LED and part ofthe reflective optical element is arranged on the opaque supportsubstrate 155. In comparison to FIG. 8A, the reflective rear surface 43may be formed on the substrate 155. Transmissive material 40 and walls49 may be formed on the surface of the substrate 155. Advantageouslythickness may be reduced. Further thermal expansion differences betweenthe substrate 155 and reflective optical element array 103 may bereduced, achieving increased uniformity.

FIG. 9A is a schematic diagram illustrating in side view a furtherstructure of reflective optical element 102, aligned micro-LED 3 andaligned wavelength conversion element. In comparison to FIG. 6A, thereflective light input structure 44 comprises a single tilted surface.Such a structure of reflective optical element 102 may be moreconveniently tooled and/or reliably replicated than the structure ofFIG. 6A. Further, only one well 70 may be provided reducing cost andcomplexity of forming wavelength conversion elements 20 in the wells.FIG. 9A further illustrates that the reflective light input structure 44may form a wall 49 of the reflective optical element 102.

Features of the arrangement of FIG. 9A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

FIG. 9B is a schematic diagram illustrating in front view alignment ofreflective optical elements 102, micro-LEDs 3 and wavelength conversionelements 20R, 20G arranged to provide a delta pixel pattern and usingthe reflective optical element 102 of FIG. 9A. Advantageously imageappearance may be improved for moving images. Features of thearrangement of FIG. 9B not discussed in further detail may be assumed tocorrespond to the features with equivalent reference numerals asdiscussed above, including any potential variations in the features.

It would be desirable to provide phosphors for wavelength conversionmaterials in high resolution display.

FIG. 10A is a schematic diagram illustrating inside view and FIG. 10B isa schematic diagram illustrating in top view operation of a reflectiveoptical element 102, a micro-LED 3 and aligned wavelength conversionelement and wherein the wavelength conversion material comprises amaterial 400 with relatively large particle size compared to the size ofthe micro-LED 3. The material 400 may comprise one or more phosphors.

In a cell phone application, a typical sub-pixel size is 15×45micrometres for example. Phosphor materials can provide efficientwavelength conversion but are typically produced by grinding to smallparticles 400, typically of size 5 micrometres or greater. The wells 70arranged on the reflective side 43 of the reflective optical element 102can confine such particles in a binding material 80 such as a siliconeand provide illumination of such particle sized phosphors for smallpixel pitches, that is at off-axis angles, thus increasing the effectivearea of the phosphor to incident illumination, even for low packingdensities that result from the large particle 400 size.

Advantageously a high-resolution colour display 100 may be provided withhigh conversion efficiency and colour gamut. Further phosphors are nottypically sensitive to oxygen and water in comparison to quantum dotmaterials, and such devices can achieve long lifetime. Further thephosphor particles 400 are remote from the micro-LED 3 and so phosphoroperating temperature is reduced and phosphor efficiency may beincreased.

The reflective optical element 102 may comprise air gap 50 between thewalls 49, reducing cost and complexity of assembly. Input light rays 110from the respective aligned at least one micro-LED 3 is input through atleast one light input region 35 of the light transmission opening 133and is output through light output regions 37 a, 37 b different from thelight input region 35 of the light transmission opening 133 afterincidence on at least one reflective rear surface 43 and the respectivealigned wavelength conversion element 400.

FIG. 10C is a schematic diagram illustrating in side view operation of areflective optical element 102, a micro-LED 3 and aligned wavelengthconversion elements wherein the wavelength conversion material comprisesa large particle size material 400 and wherein the micro-LED 3 isprovided on a light-transmissive material 40 provided between thereflective rear surface 43 and a light transmissive front surface 33. Incomparison to the arrangement of FIG. 10A, the micro-LED 3 may bealigned on the reflective optical element 102 during fabrication, andalignment of separate substrates are reduced, advantageously reducingcost and complexity.

Features of the arrangements of FIGS. 10A-C not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

It would be desirable to provide a directional output from the display100.

FIG. 11A is a schematic diagram illustrating in front view an array ofreflective optical elements 102 and aligned micro-LED 3 array whereinthe width of the output of the reflective optical elements 102transparent output region is reduced. Input light 110 is confined withinthe width 192 of the output region of the reflective optical element102, whereas no light is emitted in the region of the light blockingelement 5. Thus, a narrow width pixelated light source is provided byeach reflective optical element 102.

FIG. 11B is a schematic diagram illustrating in top view an array ofreflective optical elements 102 wherein the width 192 of the output ofthe reflective optical elements 102 transparent output region is reducedand arranged in alignment to an array of collimating catadioptricoptical elements 38 with input width 712.

A catadioptric optical element 38 is aligned with the output of eachreflective optical element 102 with width 192 that is the same or lessthan the width 712 of the input of the catadioptric optical element 38.

Illustrative light ray 190 is output from the reflective optical element102 at a high output angle with respect to the normal direction of thedisplay 100. Such a light ray is directed through the catadioptricoptical element 38 by total internal reflection and/or refraction suchthat it is output at angles close to the normal direction. Features ofthe arrangements of FIGS. 11A-B not discussed in further detail may beassumed to correspond to the features with equivalent reference numeralsas discussed above, including any potential variations in the features.

FIG. 11C is a schematic diagram illustrating in top view viewing of adirectional display by a primary user and an off-axis snooper for thedisplay 100 of FIG. 11B.

The angular size 304 of the light cone from the reflective opticalelement 102 is reduced by the catadioptric optical element 38.Advantageously the head on luminous intensity is increased and displayefficiency for a head on user 300 enhanced.

Further a privacy display as illustrated in FIG. 11C may be providedsuch that snooper 302 cannot see the displayed image. Such privacydisplays may also provide low stray light for off axis operation, forexample in night time use and automotive display.

A directional output comprises an optical output luminous intensitydistribution that is non-Lambertian to achieve higher efficiency forhead-on users. Such directional distributions typically have solid angleprofiles that have a full width for half maximum luminance in at leastone axis that is less than 50 degrees for a wide-angle mode of operationand less than 30 degrees for a privacy or stray light mode of operation.

It would be desirable to provide a switchable privacy display.

FIG. 11D is a schematic diagram illustrating in top view an array ofreflective optical elements and aligned micro-LED array aligned to anarray of collimating catadioptric optical elements 38 and an array oflinear waveguides 39 to provide a switchable privacy colour display 100.

Switchable privacy displays comprising micro-LEDs are described inPCT/GB2018/050893 and incorporated herein in its entirety by reference.

In operation, light from reflective optical elements 102 a are alignedto catadioptric optical elements 38 to provide the narrow angle size 304light cone as illustrated in FIG. 11C.

Light from reflective optical elements 102 b is aligned to linearwaveguides 39 that are arranged between the catadioptric opticalelements 38. Light rays 191 are provided that have substantially thesame directional distribution as the light ray in the reflective opticalelement 102 b. Returning to the description of FIG. 11C, light cone 306may be provided by illuminating micro-LEDs 3 aligned to reflectiveoptical elements 102 b. Advantageously a display may be switched betweena narrow angle mode for power savings and privacy operation; and awide-angle mode for multiple users and higher image uniformity.

Features of the arrangement of FIG. 11D not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

The structure and operation of catadioptric optical element 38 will nowbe described further.

FIG. 11E is a schematic diagram illustrating in top view the structureand operation of a catadioptric optical element and aligned reflectiveoptical element.

The catadioptric optical element 38 comprises in at least onecatadioptric cross-sectional plane through its optical axis 711 a firstouter surface 746 a and a second outer surface 746 b facing the firstouter surface 746 a. The first and second outer surfaces 746 a. 746 bcomprise curved surfaces.

The first and second outer surfaces 746 a. 746 b extend from a first end707 of the catadioptric optical element 38 to a second end 708 of thecatadioptric optical element 38, the second end 708 of the catadioptricoptical element 38 facing the first end 707 of the catadioptric opticalelement 38;

The distance 712 between the first and second outer surfaces 746 a, 746b at the first end 707 of the catadioptric optical element 38 is lessthan the distance 714 between the first and second outer surfaces 746 a,746 b at the second end 708 of the catadioptric optical element 38.

At least one transparent inner surface 742, 744 is arranged between thefirst and second ends 707, 708 and between the first and second outersurfaces 746 a. 746 b.

The reflective optical element 102 may be positioned between the firstend 707 of the catadioptric optical element 38 and the at least onetransparent inner surface 742, 744 of the catadioptric optical element38 and aligned with the catadioptric optical element. For example in thecross sectional plane the centre of the reflective optical element 102may be aligned with the optical axis 711 of the catadioptric opticalelement. In the present disclosure the terminology “at the first end of”the catadioptric optical element includes, for example, the micro-LEDbeing a small amount under the first end 707, in the same plane as theend 707 of the catadioptric optical element 38, or in the vicinity ofthe end 707, or in the proximity of the end 707 or adjacent the end. Ineach case this may include aligned with the optical axis of thecatadioptric optical element. The above description can be applied toall the embodiments.

A catadioptric optical system uses both reflection and refraction oflight. Further, a catadioptric optical system is one where refractionand reflection are combined in an optical system, usually via lenses(dioptrics) and curved mirrors (catoptrics). Catadioptric opticalelements can include RXI optical elements that achieve ray deflectionsby refraction (R), reflection from metals (X), and total internalreflection (I).

The first and second outer surfaces 746 a. 746 b each comprise curvedsurfaces that extend from a first end 707 of the catadioptric opticalelement to the second end 708 of the catadioptric optical element 38,the second end 708 of the catadioptric optical element facing the firstend 707 of the catadioptric optical element 38. Further the transparentinner surface 742, 744 comprises at least one curved surface 742. Theexterior angle 715 between the first end 707 and the first outer surface746 a at the first end 707 may be less than the exterior angle 717between the first end 707 and the first outer surface 746 a at thesecond end 708. Further the exterior angle between the first end 707 andthe second outer surface 746 b at the first end 707 is less than theexterior angle between the first end 707 and the second outer surface746 b at the second end 708.

Advantageously collimated light may be provided with a directional lightoutput distribution that has a narrow cone angle.

The catadioptric optical element 38 may be arranged to providesubstantially collimated output light from the reflective opticalelement 102 for light that is incident on the curved outer surfaces 746a. 746 b and the at least one of the transparent inner surface 744 whichmay have positive optical power. Further at least one of the transparentinner surfaces 744 may have zero optical power. Advantageously surfaces744 may be conveniently provided during tooling and moulding steps ofmanufacture. Further, such surfaces may cooperate to provide collimatedlight for all light rays from reflective optical element 102 over a highoutput solid angle.

Thus some of the light output illustrated by ray 718 of reflectiveoptical element 102 of the plurality of reflective optical elements 102is transmitted by the at least one transparent inner surface 744 beforeit is reflected at the first or second outer surfaces 746 a. 746 b anddirected into the first directional light output distribution 120; andsome of the light output illustrated by ray 716 of reflective opticalelement 102 of the plurality of reflective optical element 102 istransmitted by the at least one transparent inner surface 742 anddirected into the first directional light output distribution 120without reflection at the first or second outer surfaces 746 a. 746 b.

In at least one cross sectional plane, the present embodiments provide areduction in the width of the output directional light outputdistribution to provide directionality with a directional light outputdistribution (as described by solid angle Ωout) that is smaller than theinput directional light output distribution (as described by solid angleΩin) by the catadioptric optical element.

FIG. 12A is a schematic diagram illustrating an addressing system forthe plurality of LEDs. The electrodes 211, 213 of FIG. 3B for each ofthe micro-LEDs 3 of the plurality micro-LEDs 3 are respectivelyconnected to one column addressing electrode 212 and one row addressingelectrode 210 to form a matrix. In this embodiment an array of currentsources 816 is used to drive the addressing electrodes 212. The voltageon each of the row electrodes 210 is pulsed in sequence to scan oraddress the array of micro-LEDs 3. A current source 816 may be providedfor each column electrode 212 or may be time multiplexed (shared)amongst a set of column electrodes 212. The micro-LEDs 3 have arelatively sharp voltage vs. current curve and can be operated with veryshort pulses without cross-talk between them. The array of micro-LEDs 3forms an addressable display without the need for additional activecomponents such as TFTs or integrated circuits at each pixel. However,all the energy to illuminate the micro-LEDs must be provided during theaddressing pulse. Advantageously the addressing matrix is simple and lowcost.

It would be desirable to reduce the peak LED current while maintaininglight output levels. Returning to the description of FIG. 7F, drivecircuit 820 and additional addressing electrode 213 are illustrated inembodiments wherein circuits 820 at the pixels can achieve extendeddrive time for each pixel.

FIG. 12B is a schematic diagram illustrating another addressingembodiment for the plurality of LEDs. The micro-LEDs 3 of the pluralitymicro-LEDs 3 are addressed by column addressing electrodes 212 and rowaddressing electrodes 210 to form a two-dimensional matrix. For clarityonly one micro-LED 3 and one column electrode 212 and one row electrode210 of the matrix is shown. FIG. 12B differs from FIG. 12A in that eachmicro-LED 3 has associated with it an integrated circuit 808 whichincludes a storage or memory or latching function. The integratedcircuit 808 may be an analog or digital circuit and may be embodied as aseparate chip located using a method that is similar to the micro-LED 3location method or may be embodied with TFTs. The integrated circuit 808may be provided with one or more additional supply potentials V1, V3.The drive circuit 820 comprises integrated circuit 808. When the rowelectrode 210 is pulsed the clock input 810 of integrated circuit 808stores the column electrode 212 voltage connected to the Data input 812.The output 814 of the integrated circuit 808 drives the micro-LED 3. Theother end of the micro-LED is connected to supply potential V3.Depending on the design of integrated circuit 808 the potential V1 maybe different to V3. The integrated circuit 808 may include a voltage tocurrent converter. The potential V3 on electrode 213 and the anode andcathode connections of the micro-LED 3 may be configured so that themicro-LED is forward biased and emits light. The integrated circuit 808provides drive to the micro-LED 3 for longer than the duration of theaddressing pulse on row electrode 210 and the peak current drive to themicro-LEDs 3 is reduced. Advantageously the peak current in eachmicro-LED 3 is reduced.

FIG. 12C is a schematic diagram illustrating another addressingembodiment for the plurality of LEDs. The micro-LEDs 3 of the pluralitymicro-LEDs 3 are addressed by column addressing electrodes 212 and rowaddressing electrodes 210 to form a 2-dimensional matrix or array. Drivecircuit 820 comprises TFT 806, amplifier 804 and capacitor 818. In thisembodiment row electrodes 210 is connected to the gate of TFT 806 andwhen the row addressing electrode 210 is pulsed, the data from columnaddressing electrode 212 is stored on capacitor 818. Capacitor 818 maybe small compared to that typically used in a matrix to drive an LCDpanel and may be provided by the input capacitance of amplifier 804. Theamplifier 804 may drive one or more micro-LEDs 3. Amplifier 804 may beprovided with 1 or more supply voltages (not shown). Amplifier 804 mayinclude a voltage to current converter circuit. V1 may be a ground orreference potential. The voltage output from amplifier 804 must begreater than voltage V3 on electrode 213 by the combined forward voltagedrop (Vf) of the micro-LED 3 in order for the micro-LED 3 to illuminate.

The time for driving each micro-LED 3 is increased. Advantageouslycurrent crowding may be reduced and device efficiency improved.

Operation of LEDs and micro-LEDs 3 will now be described.

FIG. 13A is a schematic diagram illustrating in side view luminousintensity emission profile from a macroscopic LED 303 of the sizetypically comprised in a packaged LED, for example 0.5×0.5 mm or larger.

FIG. 13B is a schematic diagram illustrating in side view luminousintensity emission profile from a micro-LED 3 mounted on a backplanesubstrate 447 and arranged to emit light away from the backplanesubstrate 447.

After light emission 440 within macroscopic LEDs 303 and micro-LEDs 3,the high refractive index of gallium nitride causes light guiding withinthe chip.

For macroscopic LEDs 303, surface roughening can provide top surfacelight extraction and such LEDs 330 typically output a substantiallyLambertian luminous intensity profile 442 from its top surface, withlower level luminous intensity profiles 404 from edge emission.

However, scaling to micro-LED sizes surface roughening has less impacton output by top surface scatter and so the proportion of light that isoutput by edge emission increases so that top surface luminous intensityprofile 446 reduces in comparison to edge emission luminous intensityprofile 448. Such increased edge emission from micro-LEDs can reduce thelight output efficiency for desirable directions of output (such as inthe normal direction) for known micro-LED surface mounting methods.

FIG. 14 is a schematic diagram illustrating in side view variation inoptical path length through a small particle size wavelength conversioncoating layer 450.

Such coating 450 may have a different thickness to light rays 452 thatare directed in the surface normal direction in comparison to rays 454that are emitted in a lateral direction. Such a different thickness canprovide an undesirable colour change of output with viewing direction.Further light may be lost in thicker layers of wavelength conversionmaterials, reducing efficiency. Further, accurate methods with verysmall volumes of material may be used to provide control of colouroutput.

Further heating from the micro-LED 3 can degrade the efficiency andlifetime of the wavelength conversion material 450.

Advantageously the present embodiments achieve a uniform illuminationfor light rays 452, 454 from the micro-LED 3 onto the wavelengthconversion elements 20 and reduce the operating temperature of thewavelength conversion materials.

Arrangements of phosphor particles 400 with micro-LEDs in known colouroutput micro-LED 3 will now be described.

FIG. 15A is a schematic diagram illustrating in side view output from amicro-LED comprising wavelength conversion coating with large particlesizes; and FIG. 15B is a schematic diagram illustrating in front viewoutput from a micro-LED comprising wavelength conversion coating withlarge particle sizes. As described hereinbefore, the particles 400 ofphosphor may have sizes that are similar to the sizes of micro-LEDs 3for high resolution cell phone applications. Thus, light may be lostbetween particles 400 and colour gamut may be reduced and inconsistentcolour output achieved across an array of micro-LEDs 3. Advantageouslythe present embodiments achieve a uniform illumination for phosphormaterials as illustrated in FIGS. 10A-B.

Features of the arrangements of FIGS. 15A-B not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

Manufacturing methods for array 103 of reflective optical elements 102and colour display 100 will now be described.

FIGS. 16A-G are schematic diagrams illustrating in side view a method toform an array of reflective optical elements 102 and aligned micro-LEDarray. For illustrative clarity a representative pair of reflectiveoptical elements 102 of the array 103 is illustrated, however the array103 may in practice comprise millions of reflective optical elements102, numbering the total number of colour sub-pixels in a display, or inmany displays.

In a first step as illustrated in FIG. 16A, a tool 150 may be provided,for example by diamond turning of a metal tooling blank.

In a second step as illustrated in FIG. 16B, the body 47 may be formedby means of embossing, casting, injection moulding against the tool 150or other known replication methods, and the tool 150 removed.

In a third step as illustrated in FIG. 16C a coating 41 may be appliedto the body 47. The coating may be an evaporated, sprayed or sputteredmetal coating or may comprise a coated white material, formed forexample by dip or spray coating. Alternatively, the body may be formedin a reflective white material such as CEL-W epoxy marketed by HitachiChemical and the third step omitted.

In a fourth step as illustrated in FIG. 16D, wavelength conversionmaterials may be applied on to the surface or in to wells 70 of thereflective surface to provide wavelength conversion elements 20. Thewells 70 provide a defined location for the wavelength conversionmaterial when deposited from a liquid solution for example by means ofink-jet printing. After deposition the liquid carrier is evaporated toleave the wavelength conversion material.

In a fifth step as illustrated in FIG. 16E, a filler material 40 isprovided between the reflective surface and a mould and cured to provideoutput surface 33. The reflective optical elements 102 in a firstdirection have a separation s2.

In a sixth step as illustrated in FIG. 16F micro-LEDs 3 and addressingelectrodes (not shown) are arranged on the surface 33, where theseparation s1 of the micro-LEDs 3 in the same first direction is thesame as the separation s2.

In a seventh step as illustrated in FIG. 16G, light blocking layer 5 isformed on the micro-LEDs 3. The light blocking layer may be a metal or ablack resin. In the embodiments that the light blocking layer 5 is ametal, it may also function as part of the pixel addressing circuitryused to address and control the micro-LEDs 3.

Features of the arrangements of FIGS. 16A-G not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

A highly parallel method of manufacture for colour display 100 will nowbe described.

FIGS. 17A-17F are schematic diagrams illustrating in side view a methodto form in parallel an array of displays 100 comprising arrays ofreflective optical elements 102 and aligned micro-LED 3 arrays.

As illustrated in FIG. 17A, a monolithic LED wafer 2 that may be galliumnitride for example may be formed on a substrate 4 that may be sapphire,silicon or silicon carbide for example.

As illustrated in FIG. 17B, a non-monolithic army of micro-LEDs 3 may beextracted from the monolithic wafer 2 to provide micro-LEDs 3 a, 3 bwith separation s1.

As illustrated in FIG. 17C, micro-LEDs 3 a, 3 b may be arranged onsubstrate 52 in alignment with addressing electrodes and other optical,electrical and thermal management elements (not shown) such thatseparation s1 is preserved.

As illustrated in FIG. 17D, the substrate 52 may be aligned with aplurality of micro-LEDs with separations s1.

As illustrated in FIG. 17E, the substrate 52 and array 103 are alignedso each micro-LED 3 is aligned with a respective reflective opticalelement 102.

It would be desirable to provide multiple illumination apparatuses fromlarge areas of aligned optical elements. As illustrated in FIG. 17F, thesubstrate 52 and array 103 may be provided at substantially larger areathan the area of an individual display. Thus, various different colourdisplays 100 may be extracted with different areas and shapes 600, 602,604, 606.

Advantageously large numbers of displays 100 may be formed over largeareas using small numbers of extraction steps, while preservingalignment to a respective array of optical elements.

Further device seal lines 601 may be provided at the edge of eachelement to provide hermetic sealing of the optical elements and reducedust and other material ingress into the optical elements during use.

Advantageously manufacturing cost and complexity can be reduced, andreliability during use increased.

Features of the arrangements of FIGS. 17A-F not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

The words “substantially” and “approximately”, as may be used in thisdisclosure provide a tolerance which is accepted in the industry for itscorresponding word and/or relativity between items. Such anindustry-accepted tolerance ranges from zero to ten percent andcorresponds to, but is not limited to, lengths, positions, angles, etc.Such relativity between items ranges between approximately zero to tenpercent.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiment may include or work with a variety oflighting, backlighting, optical components, displays, tablets and smartphones for example. Aspects of the present disclosure may be used withpractically any apparatus related to displays, environmental lighting,optical devices, optical systems, or any apparatus that may contain anytype of optical system. Accordingly, embodiments of the presentdisclosure may be employed in displays, environmental lighting, opticalsystems and/or devices used in a number of consumer professional orindustrial environments.

It should be understood that the disclosure is not limited in itsapplication or creation to the details of particular arrangementsillustrated, because the disclosure is capable of other embodiments.Moreover, aspects of the disclosure may be set forth in differentcombinations and arrangements to define embodiments unique in their ownright. Also, the terminology used in this disclosure is for the purposeof description and not of limitation.

While embodiments in accordance with the principles that are disclosedherein have been described, it should be understood that they have beenpresented by way of example only, and not limitation. Therefore, thebreadth and scope of this disclosure should not be limited by any of theexemplary embodiments described, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. In addition, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

The section headings herein are included to provide organizational cues.These headings shall not limit or characterise the embodiments set outin any claims that may issue from this disclosure. To take a specificexample, although the headings refer to a “Technical Field,” the claimsshould not be limited by the language chosen under this heading todescribe the field. Further, a description of technology in the“Background” is not to be construed as an admission that certaintechnology is prior art to any embodiment in this disclosure. Neither isthe “Summary” to be considered as a characterization of the embodimentsin issued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there ismerely one point of novelty in this disclosure. Multiple embodiments maybe set forth according to the limitations of the multiple claims issuingfrom this disclosure, and such claims define the embodiments, and theirequivalents, that are protected by them. In all instances, the scope ofclaims shall be considered on their own merits in the light of thisdisclosure and should not be constrained by the headings used in thisdisclosure.

1-48. (canceled)
 49. A colour display apparatus comprising: a pluralityof micro-LEDs arranged in a micro-LED array, wherein the plurality ofmicro-LEDs are micro-LED die chips, wherein each micro-LED die chip hasa width or diameter that is at most 150 micrometers; a plurality ofreflective optical elements arranged in a reflective optical elementarray; an output substrate; and a plurality of wavelength conversionelements arranged in a wavelength conversion array, wherein each of theplurality of wavelength conversion elements is arranged to receive lightemitted by one or more micro-LEDs of the plurality of micro-LEDs,convert the received light into light of a different colour wavelengthband, and output the light of a different colour wavelength band fordisplay, wherein each of the plurality of reflective optical elements isarranged to: receive at least part of the light of a different colourwavelength band output by a wavelength conversion element of theplurality of wavelength conversion elements; and reflect the received atleast part of the light of a different colour wavelength band towardsthe output substrate for display.
 50. The colour display apparatus ofclaim 49, further comprising a reflective light input structure arrangedto reflect at least part of the light emitted by a micro-LED of theplurality of micro-LEDs towards a wavelength conversion element of theplurality of wavelength conversion elements.
 51. The colour displayapparatus of claim 50, wherein the reflective light input structure isfurther arranged to allow at least part of the light output by thewavelength conversion element to pass therethrough towards the outputsubstrate.
 52. The colour display apparatus of claim 50, wherein thereflective light input structure is located between the micro-LED andthe output substrate.
 53. The colour display apparatus of claim 49,wherein each of the plurality of reflective optical elements comprises areflective rear surface and reflective walls extending from thereflective rear surface, the reflective rear surface and reflectivewalls defining a space therebetween.
 54. The colour display apparatus ofclaim 53, wherein each of the plurality of wavelength conversionelements is located within the space defined by the reflective rearsurface and reflective walls of a respective reflective optical element.55. The colour display apparatus of claim 49, wherein the plurality ofmicro-LEDs are located in substantially the same plane as the pluralityof wavelength conversion elements.
 56. The colour display apparatus ofclaim 49, further comprising a support substrate, wherein the pluralityof micro-LEDs are arranged on the support substrate.
 57. The colourdisplay apparatus of claim 56, wherein the support substrate is opaque.58. The colour display apparatus of claim 57, wherein the supportsubstrate comprises opaque electrode materials and other controlelectronic components.
 59. The colour display apparatus of claim 56,wherein the support substrate comprises thermally conductive regionsand/or layers arranged to achieve reduced semiconductor junctiontemperature during operation of the plurality of micro-LEDs.
 60. Thecolour display apparatus of claim 49, further comprising a plurality oflight blocking elements, wherein each light blocking element is alignedwith a respective micro-LED of the plurality of micro-LEDs.
 61. Thecolour display apparatus of claim 60, wherein each micro-LED of theplurality of micro-LEDs is located on its respective light blockingelement.
 62. The colour display apparatus of claim 60, wherein eachmicro-LED of the plurality of micro-LEDs is at least partially embeddedin its respective light blocking element.
 63. The colour displayapparatus of claim 60, further comprising a support substrate, whereineach of the plurality of light blocking elements is arranged between thesupport substrate its respective micro-LED.
 64. The colour displayapparatus of claim 60, wherein each micro-LED of the plurality ofmicro-LEDs is arranged between its respective light blocking element anda reflective light input structure.
 65. The colour display apparatus ofclaim 56, wherein each of the plurality of reflective optical elementsis either spaced apart from the support substrate or arranged on thesupport substrate.
 66. The colour display apparatus of claim 49, whereineach of the plurality of wavelength conversion elements comprises aphosphor or a quantum dot material.
 67. The colour display apparatus ofclaim 49, wherein the output substrate comprises a transparentsubstrate, a retarder, and a polariser.
 68. A smart phone or tabletcomprising the colour display apparatus of claim 49.